Current transformers are frequently employed to monitor power line AC current level. The current transformer usually includes a magnetic core having a relatively high magnetic permeability and a primary winding coupled to a line being monitored. A multiturn secondary winding is wound on the core to drive relatively high impedance circuitry that does not substantially load the primary winding. Such structures are used to monitor current for current measuring purposes, as well as to assist in determining real and reactive power, and to drive watt hour meters.
In some situations, a DC level is imposed on the primary winding of the current transformer. For example, half wave rectifiers are sometimes connected directly in the primary winding in attempts to defeat the accuracy of the current measurement. In other instances, DC rectification loads are connected to the line and coupled to the current transformer primary winding.
The DC level in the primary winding biases the high permeability magnetic core in one direction, so that the core has a tendency to become saturated to a greater extent during half cycles of one polarity than during half cycles of the other polarity. Even if the core is not driven into saturation during half cycles of either polarity, the amplitude of the voltage induced in the secondary winding differs during opposite polarity half cycles with a core having a DC bias superimposed thereon.
I am aware of two different prior techniques for detecting the asymmetric responses during opposite polarity half cycles of a current transformer secondary winding having a primary winding subjected to a DC level. In the first technique, referred to as differential peak detection, a derived control signal has an amplitude dependent upon the difference in positive and negative peaks of the secondary current. In the second, zero crossing detection technique, the control signal has an amplitude dependent upon the differential duration of the positive and negative half cycles of a replica of the current in the secondary winding. The control signal is derived as a DC voltage which is applied to a compensating winding wound on the current transformer. Ideally, the DC current applied to the compensating winding tends to restore the DC magnetic flux of the transformer core to the same level it would have had if no DC were imposed on the primary winding of the transformer.
The differential peak detection technique, as exemplified by Milkovic, U.S. Pat. No. 4,255,705, is not always effective because transient effects in the transformer affect the core magnetizing inductance. If, e.g., a positive half wave rectified current is applied to the current transformer, the secondary voltage has positive and negative peaks with similar magnitudes. The magnitudes are about the same because the primary winding half wave rectified positive current is converted into an AC secondary voltage waveform having a first segment that resembles a half wave rectified sine wave and a second segment including a negative peak followed by an exponential ramp. When the primary winding current initially goes positive at the beginning of a positive sinusoidal half cycle of the rectified current, the secondary winding output voltage is negative. The secondary voltage then increases to a positive peak value, which occurs approximately simultaneously with the peak value of the primary winding current. In response to the primary winding current returning to zero, the secondary winding voltage drops suddenly to a negative peak value which is approximately equal to the secondary positive peak value. The exponential ramp of the second segment then is derived until the primary winding starts to go through another sinusoidal half cycle. The first and second segments occur while the primary half wave rectified current has finite and zero values, respectively.
The differential peak detection technique is subject to significant errors because the positive and negative peak values have approximately the same amplitude for the frequently encountered situation of a half wave rectified sine wave current applied to the current transformer. Very large percentage errors occur when two values having approximately the same amplitude are compared. Hence, the differential peak detection technique is subject to inaccuracies which cannot be tolerated in a high probability situation.
The zero crossing detection technique is subject to inaccuracies if the amplitudes of a sinusoidal waveform and a half wave rectified sinusoidal waveform simultaneously applied to the primary winding are approximately the same. If, e.g., a sinusoidal waveform having the same amplitude as a half wave rectified waveform is applied to the current transformer primary, the difference in zero crossing periods of the half secondary winding current is reduced approximately by a factor of two.